The spatial and temporal evolution of thermal stress after granite emplacement and its influencing factors

Granitic magmas are genetically associated with magmatic-hydrothermal deposits and oil and gas reservoirs. The emplacement of granitic magmas into cooler rocks produced thermal anomaly and thermal stress, yet systematic studies on the spatial and temporal evolution of thermal stress still need to be completed. Previous numerical modeling often used rocks' linear thermal expansion coefficient at room temperature, but this parameter is highly temperature-dependent and reaches much higher levels at high temperatures. Therefore, the magnitude of the thermal stress caused by magma emplacement needs to be re-examined. A series of numerical experiments were carried out to investigate how the surrounding rock's lithology (granite or carbonate rocks), Young's modulus, thermal parameters, and the depth of magma emplacement affect the thermal stress generated by the magma in the overlying surrounding rocks.

Because the magma cools and eventually reaches thermal equilibrium with the surrounding strata, numerical simulation is one of the common methods to examine the thermal stress after magma emplacement quantitatively. This article used FLAC^3D software to simulate the thermal stress caused by the emplacement of granitic magma into the upper crust. The differential equations we solved include the thermal conduction and linear thermoelasticity equations. The models' thermal field influences the stress field through temperature difference and the linear thermal expansion coefficient. However, changes in the stress field do not affect the thermal field, i.e., one-way coupling between the thermal field and the stress field.

(1) Heat transfer is quicker on wallrocks with high thermal conductivity, causing a faster change in thermal stress. Compared to the high-thermal-conductivity case, the same thermal stress can be produced on wallrocks with a lower thermal conductivity after a more extended period of magma cooling. (2) The thermal stress produced by the surrounding rock's Young's modulus of 80 GPa is higher than the surrounding rock's Young's modulus of 60 GPa and 40 GPa. (3)The thermal stress simulated in the article is an order of magnitude larger than those generated using the linear coefficient of thermal expansion at room temperature. The thermal stress induced by granite surrounding rocks is nearly 30 MPa higher than that induced by carbonate rocks. (4) The thermal stress decreases with increasing distance from magma, approaching the initial stresses at nearly 2 km. (5) When the emplacement depth is shallow, both initial temperature and initial stress are lower than those in deeper emplacements; The magma room cools faster at shallow depths. Because the initial temperature of magma is the same, shallow emplacements will produce higher thermal stresses on overlying surrounding rocks.

The modeling results indicate that the thermal conductivity of surrounding rocks influences the change rate of thermal stress through the heat transfer rate. The thermal stress increases with the surrounding rock's Young's modulus. Since the average Young's modulus of granites is greater than that of carbonate rocks, the thermal stress on granite is greater than that on carbonate rocks. Either granites or carbonate rocks at high temperatures have a thermal expansion coefficient about one order of magnitude greater than that at room temperature, resulting in thermal stress of up to 100 MPa. The temperature of the surrounding rock gradually increases after the granite magma emplacement, corresponding to the increasing thermal stress. The thermal stress decreases with increasing distance from magma, exerting no influence on the initial stress of host rocks above 2 km of the granitic magma. When the magma emplacement is shallow, the combination of high thermal stress and low initial stress is more conducive to the formation and expansion of fractures in overlying surrounding rock.

The results of numerical simulations reveal that the thermal stresses generated by magma emplacement can affect the stress field 2 km above the magma. These localized and short-lived thermal stresses may fracture the overlying rocks, providing transport channels or ore-bearing spaces for later hydrothermal fluids.